During the process of export of maltose-binding protein to the periplasm of Escherichia coli, the leader peptide is involved in at least two steps. The presence of the leader portion of maltose-binding protein was shown to be necessary to mediate initial binding of the precursor to the membrane. However, the presence of a mutationally altered leader which does not sustain export in vivo was sufficient to allow this interaction. Thus, the defect in export which is manifested in vivo by this mutational substitution occurs at a step that follows membrane association, most likely the translocation step. Translocation occurs at discrete sites that are not uniformly distributed over the cytoplasmic membrane. A large proportion of the membrane involved in translocation has a higher density than that of bulk cytoplasmic membrane.